I don't think it is possible. Are they "testing the employees", to see how they answer the phone? Are they not planning on identifying themselves during the conversation? After one or two calls, word would get around the office that if XXXX shows up on your call display, that it's "John" in accounting calling.
People might end up doing what residential customers do when a blocked number calls, just not answer the phone if the caller is going to "play games".